The helicopters were shot down with surface-to-air missiles, which, according to the Ukrainian Security Service, shows Russia is, at the very least, supplying the separatists with weapons. One pilot survived and was reportedly captured.

"The terrorists opened fire at Ukrainian units with some heavy guns, including grenade launchers and portable air defense systems," Interior Minister Arsen Avakov said in a post on his official Facebook page.

Vyacheslav Ponomaryov, a pro-Russian separatist sand the self-declared mayor of Slovyansk, said the city was under attack.

"We are being stormed, we have got casualties. I'm asking children, women and the elderly not to leave their homes and I ask armed men to provide us all the assistance they can," he said. "I think we will be able to successfully stand up for our city. Thank you for your attention, thank you for your assistance, we will win."

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Meanwhile, several journalists were detained for several hours Friday by pro-Russian groups as they traveled to Slovyansk, including a CBS News crew.

Correspondent Clarissa Ward, her producer Erin Lyall, and two male CBS News employees were held for several hours after pro-Russian militants at a checkpoint near the city of Horlivka detained and blindfolded them.

"We were blindfolded with cloth and masking tape really quite tightly around our heads so we couldn't see anything at all," Ward told "CBS This Morning" over the phone not long after they were freed.

She said they were bundled into a van and driven to another location where their captors were "quite rough and sharp with us. One woman was shouting at us not to speak."